Radio terminals that communicate with orbiting satellites are gradually coming into commercial use. These terminals can communicate bi-directionally with direct broadcast TV and telephone satellite systems for providing communication services directly to households via dish antennas. In such communication systems, a satellite uses polarized radio waves (beams) to communicate with terminals located in a service area.
Satellite communication systems commonly use various types of radio wave polarization. A radio wave may be polarized linearly, for example, vertically or horizontally, or it may be polarized non-linearly, for example, elliptically or circularly. The polarization of a radio wave is defined by the direction in which electric vectors are aligned during at least one full cycle. Generally, both the magnitude and the direction of the electric vectors vary non-linearly during each cycle. Usually, such non-linearly varying electric vectors map out an ellipse on a plain normal to the direction of propagation at a point of observation. In this case, the non-linear polarization of the radio wave is said to be elliptical. The minor-to-major-axis ratio of the ellipse, which is expressed in decibels, is called the ellipticity of the radio wave. A linearly polarized radio wave has an ellipticity of infinity, that is, the minor-to-major-axis ratio is zero. A circularly polarized wave has an ellipticity of zero dB, that is, the minor-to-major axis ratio is unity. The linearly polarized wave is, therefore, defined as a transverse electro-magnetic wave whose field vector at a point in a homogenous isotropic medium at all times lies along a fixed line. The circularly polarized wave is similarly defined as a electro-magnetic wave for which the electric and/or magnetic field vector at a point describes a circle.
A circularly polarized wave may have a right-hand circular polarization or a left-hand circular polarization. A right-hand circular polarization occurs when, for an observer looking in the direction of wave propagation, the rotation of the electric field vector in a stationary transverse plain is clockwise. Conversely, the rotation is counter-clockwise for a left-hand polarization. A circularly polarized wave may be produced by a helical beam antenna having a corresponding right-hand or left-hand sense. The circularly polarized wave may also be produced by the coexistence of two linearly polarized waves, such as a vertical and a horizontally polarized waves, each having the same amplitude but a 90.degree. phase difference between them. If the linearly polarized waves are not equal in amplitude or have a phase difference other than a 90.degree. relationship, the resulting radio wave will be polarized non-linearly. If for example, the amplitude of the vertical polarized wave is zero, the resulting wave is linearly polarized with a horizontal orientation. Further, if the two waves have equal amplitude but zero degree phase difference, the resulting wave is linearly polarized with a 45.degree. orientation.
In order to better use the limited frequency spectrum allocated for offering satellite services, two different terminals communicating with the same satellite may use the same radio wave, but different polarization. For example, the same radio wave may have a horizontal polarization for communicating a modulated signal with one terminal and a vertical polarization for communicating the same or another modulated signal with the other terminal. Thus, it becomes necessary to selectively control the wave polarization in a terminal transmitter that transmits the modulated signals according to an allocated polarization.
FIG. 1A shows a conventional transmitter 10 for transmitting radio waves, which are selectively polarized either vertically or horizontally. The transmitter 10 includes a relatively high-power amplifier 12 for amplifying a modulated signal provided by a modulated signal generator 14. A polarization switch 16, which is controlled by a control signal on line 18, selectively connects the output of the power amplifier 12 to either a vertical input 20 or a horizontal input 22 of an antenna feed 24, which radiates a polarized wave with a selected vertical polarization or horizontal polarization. Because the high-power amplifier 12 must amplify the modulated signal to a full transmit power level, for example, 8 watts, the polarization switch 16 must be selected to withstand the full brunt of such a high power. A high-power active switch with a low loss, however, is expensive. Lower cost switches, on the other hand, introduce substantial loses of up to 20% of the amplifier's power. Consequently, an amplifier with higher power, i.e., 10 watt, must be used to accommodate the loss caused by the low cost switch. Alternatively, a mechanical switch may be used to perform the switching function. In addition to being bulky, however, such switches are subject to mechanical failures.
With the success of modern processing technologies in reducing the cost of power amplifiers more than the cost of high-power switches, another conventional transmitter 26, shown in FIG. 1B, uses two separate power amplifiers 28A and 28B, instead of the single switch and power-amplifier arrangement of FIG. 1, to eliminate the losses caused by a switching arrangement. Under the arrangement of FIG. 1B, however, only one power amplifier is enabled, via a controller 29, at a time. Because one power amplifier is idle half the time, the transmitter of FIG. 1B wastes the cost of one of the high-power amplifiers at any instance of time.
Therefore, there exists a need for a low-cost transmitter that can selectively transmit differently polarized modulated signals, without any loss ar waste of transmission resources.